1. Field of the Invention
This invention relates to keyboard switches of the type that are activated by tactile depression, and more particularly a keyboard switch of the class that uses magnetic field intensity as the sensed variable to detect switch actuation.
2. Description of the Prior Art
There has been a burgeoning growth of data processing and information encoding devices that receive data and information through a keyboard input. The general class of keyboard associated with these devices comprises an ordered array of switches; each switch representing an alphanumeric or other type character, and actuated by a depressive force from the keyboard operator.
The basic switch design generally includes a reciprocating plunger that travels downward under the influence of the depressive force and is biased to an upward rest position when the force is removed. The downward travel of the plunger causes a change in the condition of a sensed variable. The change is detected by a transducer and translated into a signal. The sensed variable may be a mechanical, electrical, or magnetic quantity, and the transducer signal is usually a voltage or current.
Considering the specific case of a keyboard switch where the sensed variable is magnetic, the plunger must have some magnetic characteristic associated with it that is varied in time or space as the plunger travels downward. A straight-forward implementation of this design is to equip the plunger shaft with a permanent magnet at its lower end and position a magnetic sensor at the lower extreme of its travel to sense the increase in magnetic field intensity when the permanent magnet comes into proximity to the sensor.
The prior art has implemented this basic design in a number of variations. However, each of the various prior art designs is attended by certain, identifiable limitations that affect the cost, performance and reliability of the keyboard switch. More specifically, most of the prior art devices incorporate a relatively complex assembly of parts. A greater number of parts increases material cost and requires a plurality of assembly steps, and because the typical keyboard generally requires in excess of 40 switches, the problem of high manufacturing costs may make the selection of a certain design prohibitive. There is also the problem of tolerance build-up associated with any device having a plurality of moving, mechanical parts. Close tolerances are reflected in high manufacturing costs and limited ability to mass produce the parts. Moreover, the operation and useful life of a device will be affected by frictional and other wear characteristics inherently associated with any device having moving mechanical parts.
A good illustration of this point is the keyboard switch disclosed in IBM Technical Disclosure Bulletin, Vol. 15, No. 7, December 1972, "Magnetic Key Mechanism", M. Sulich et al. The Sulich et al device incorporates a first, cooperating pair of coaxial, annular magnets in the switch housing and plunger to bias the plunger to an upward rest position; a second, cooperating pair of coaxial, annular magnets to provide a snap-action effect upon depression of the plunger; a magnetic switch structure incorporated into the bottom of the plunger that cooperates with a magnetic sensing device mounted at the base of the switch housing; and slide bearings and pins which, along with the permanent magnets, make up a unitary plunger assembly. It is manifest from a cursory inspection of the Sulich et al or other prior art devices that the more sophisticated switch design, the more of a trade-off is required in cost and reliability.
Against this background, there is an identifiable need for a keyboard switch that is inexpensive to manufacture, reliable in operation, and provides full and consistent performance over an extended life.